She can’t divorce him guys. He hasn’t actually done anything that would warrant a divorce between nobles in noble society. Nobles have all sorts of contracts and agreements that make it difficult for them to easily divorce. Also, if the MC were to say “I know he likes my sister. He has a look in his eyes that says he doesn’t love me!”, it would only sound like she has self esteem or self confidence issues and lead to society thinking she’s mentally unwell. That would then give the hubby the excuse he would need to leave the MC and marry the sister instead.